Electron-image device



July 15, 1941.

E. G. RAMBERG ETAL ELECTRON- IMAGE DEVI CE Filed March 31, 1938 R l lk;----* 3nventovs Edward G. Ramber George (Ittomeg Patented July 15,1941 Edward G. Ramberg, Haddon Heights, and George A. Morton, Audubon,N. J assignorsto Radio Corporation ofAmerica, a corporation of DelawareApplication March 31, 1938, Serial No. 199,110

3 Claims. (01. 250-150) Our invention relates to electron-opticalsystemsof the type designed to handle extended -or entireelectron-images (as distinguished from images built up or scanned by amoving pencil or beam of electrons).

, The principal object of our invention is to provide novel methods andapparatus for correcting certain image defects peculiar to theproduction and transmission of entire electron-images.

The image defects with which our present invention is chiefly concernedare usually manifest ,in electron-microscopes, electron-telescopes,electron-image intensifiers, and similar electronimage devices, by anapparent lack of focus which persists even when the electrostatic ormagnetic focusing elements are adjusted for optimum performance.

Our invention is predicated upon theknowledgethat such image defects aredue, at least in part, to the fact that the electrons leaving a givenobject point inthe usual electron-image device are subject .to variousdegrees of convergence in passing to the next succeeding electrode,whereby someof the electrons are directed toward and impinge upon pointsother than the image point upon which it is desired that they shouldimpinge. The lack of uniformity in the convergence of the electrons weattribute to the fact that the electrons as emitted exhibit variousvelocities and velocitycomponents so that the paths of the individualelectrons are not uniformly affected by the electromagnetic andelectrostatic focusing and accelerating fields.

By changing the direction of the electrons (in the space between any twoelectrodes) we effect a change in the direction and. relative rate ofconvergence of electrons having different velocities, and-are thus ableto direct a substantial majority of the electrons from a given objectpoint to the corresponding image point on the next succeeding target orscreen electrode, whereby the above-described image defects aresubstantially obviated.

We do all this with mirrors, 1. e., with electron-mirrors.

While our invention will be described as applied to an electron-imagetube of the type disclosed in our copending application Serial No.193,134, filed February 28, 1938 (RCV D-5563), it will be apparent thatour invention is not limited to such application and that the disclosurein this respect is merely illustrative for purposes of explaining theinventive concept. I Figure 1 is a partly diagrammatic plan view of anelectron-image intensifier device inwhich is .tains animage-intensifying electrode. purpose of directing the primary-electronsinto incorporated an electron-mirror designed, positioned and arrangedin accordance with the principle of our invention to correctelectron-image defects, and g Figure 2 is a side elevational view, on areduced scale, of the device of Fig. 1, showing the device subjected toa magnetic field, as in normal operation. V

In the drawing, T designates generally a highly evacuated envelope whichhas four neck portions or extensions A, B, C and D, respectively.

The vacuous space in the main or bulbous bodyportion of the tube and inthe necks A, B, C and D is continuous. Y.

The innerwall ofthe bulbous part of the tube is provided with a coatingM of metalto which a potential may be applied through a lead 112.

This metal layer M, which constitutes an anode, extendsto the mouth ofeach of the extensions A, B,and C, and into the extension D to the pointadjacent the terminal end thereof.

Mountedin' the neck portion A of the tube is a photo-sensitivecathode Iwhich is adapted to release an entire primary-electron image in responseto the impress thereon of a light image from a source exemplified in thedrawing by an .optical lens L. The image-cathode l is preferably curvedin a known manner to correct for image defects. A series of spaced rings2, 3 and 4, and acylinder 5, arranged in the order named,

.constitute an electron-lens system which, when supplied with suitablepotentials from a source exemplified in the drawing by a voltage dividerR, will project the electrons constituting thev entire primary electronimage into the vacuous interior of the main portion of the tube. Unlesssome'force intervenes, the electrons constituting such image willcontinue to advance along the axis'of the extension A and miss theextension B which, as will more fully hereinafter appear, con For thethe extension B, a magnetic field is required which is perpendicular tothe plane containing the longitudinal axes of the extensions A, B, C andD.

. Such magnetic field may be produced in the mannerindicated in Fig. 2,i. e., by a pair of coils F, F suitably energized and supportedexteriorly' of the envelope T. g I

The primary or photoelectrons leaving the cathode l are subject to theaccelerating field produced cooperatively by the voltages appliedthrough adjustable leads 2a, 3a, 4a, etc., to the focusing rings 2, 3and l, and to the focusing cylinder 5, and anode M,through leads 5 andformed by the intersection m, respectively, which, as indicated in thedrawing, are connected to the most positive point (say, 5,000 volts) onthe voltage divider R. Primaryelectrons thus approach the open end ofthe extension B at a relatively high velocity.

Electrode II, adjacent the closed end of the neck B, is the intensifyingor electron-multiplying electrode. This electrode may comprise a silveror other metal plate, treated with caesium or equivalent substance inorder to enhance its ability to release secondary-electrons by impactexcitation. The ring-like electrodes I2, I3 and I4 and the cylinder I5comprise elements of an electron-lens system which servesto focus theprimary-electron image upon the multiplying electrode II and likewiseserves to accelerate and focus the secondary-electrons from thatelectrode into the open central portion of the tube, where they aresubjected to the influence of the magnetic field and directed into theextension C, which contains the electron-mirror that we employ in curingdefects in the electron-, image.

Extension D contains an output electrode, here shown in the form of afluorescent screen 3|, upon which the corrected image eventualb impingesto form a light-image which is visible through the transparent end ofextension D.

Considering the cathode I to be maintained at ground potential, thefocusing rings 2, 3 and 4,

.at potentials such as to produce a sharp image on the multiplyingelectrode II, e. g., each 50 volts positive with respect to the nextpreceding electrode, and the focusing cylinderfi, and anode M, to bemaintained at, say, 5000 volts positive,

with respect to the cathode, it will be apparent (since the magnet in nowise afiects thespeed of'the electrons) that theprimary electrons willapproach the extension B with. a velocity of 5000 volts because thecylinder I5 in that extension is maintained at that potential by, lead15a. The speed of the primary electrons rpassing through the cylinder I5and the focusing rings that is to say, each ring-like electrode may bemaintained at a voltage, e. g., 50 volts, higher than its next precedingelectrode while the cylinder I5 is maintained at the anode potential,

e. g., 5000 volts.

While the electrons constituting the primaryelectron image and thoseconstituting the secondary-electron image both pass through an electrodeI5 maintained at a voltage of 5000 volts positive with respect to thecathode, the departing secondary-electrons will actually be travelingmuch slower (i. e., 1000 vol-ts slower) than the enteringprimary-electrons, because the potential drop between the electrode I Iand the anode M or the cylinder I5 is less than the drop between thatelectrode andthe cathodel. Since the magnetic field has a greater effectupon slower moving secondary electrons than upon the faster movingprimary electrons, the angle formed by the intersection of theaxes ofextensions B and C should be smaller-than theangle of the axis of withthe axis of B. 1 1

It might be inferred from the foregoing that all of the electronsreleased from the secondaryelectron emitter I I, which are subject tothe accelerating potentials applied to electrodes I2 to I5, and theanode M have the same velocity. Such is not the case. Indeed, if itwere, the application of the invention to tubes of the type describedwould be without utility. Actually the electrons leaving any givenobject point on electrode II (or cathode I) will have variousvelocities, approximately 90% of which will be within, say 3 volts ofeach other. This difierence in velocity may be made up of differentcomponents,e. g., a forward component and a lateral component; both ofwhich cause the electrons to travel in paths which may be described asout of register with the path traveled by the electrons (from the sameimage point) emitted. with zero velocity. This is indicated by thecurved lines at, 11 1 and 1112 of Fig. 1. Line 0: represents the path ofan electron emitted from electrode II with zero velocity, line 1111representing the path of an electron, from the same: image point, havinga forward, velocity component of, say, volt, and a lateral velocitycomponent of, say, volt, and line 112 represents an electron from thesame point having a, forward velocity component of 1 volt and a lateralcomponent of, say, two volts.

While-in the. draw-ing the relative curvature of the electron paths a:,11/1. and 11/2 will. be understood to be exaggerated, nevertheless, inpractice, the paths of the electrons are such. that, with the usualelectron lens systems, many of the electrons from a given object pointwill miss the corresponding image. point on the target orscreenelectrode sufiiciently to give rise to the previously describedvisible aberrations or image defects.

trons having intermediate velocities are focused at slightly differentpoints.

However, we achieve :an improvement inthe image of five or more times,i. e., the circle of diffusion is reduced-by a factor of fiveor more.

While various types of electron mirrors are known, we prefer to use aconstruction comprising two spaced coaxial cylinders 2I and 22, disposedin the extension C, and between which the electrostatic field whichconstitutes the mirror is; generated. The outer cylinder 221s preferablyclosed as by a plate 22p, which, like the cathode I, may be curved tocorrect for other image defects. The leading cylinder 2-I is preferablymaintained by adjustable lead 2Ia at the same high potential as theanode M, while the second cylinder 22 is connected by lead 22a to apoint on the voltage divider R, which may be several hundred voltsnegative with respect to the image cathode I.

With the cylinders 2I and '22 thus energized, the electrons enteringthem and subject to the electrostatic field therebetween have theirdirection reversed. The departing electrons, under the coils F, F aredirected toward the extension D and focused upon the target or screenelectrode 31 in a manner hereinafter more fully described.

The effect of the mirror (i; e., the electrostatic field between 2| and22). depends upon the potentials applied to the cylinders 2| and 22. For

a given potential distribution, electronsof two different velocities aremade to focus at exactly the same point.

The efiect of the electron mirror will perhaps be better understood whenit is recalled that fast moving electrons, from an emitting object pointwhich is on the axis of an electron-optical system, will not, in passingthrough an electron lens, converge upon the axis of the system asrapidly as electrons emitted, from the same point, with low velocities.However, if an electron mirror is used in conjunction with the lens, therate of convergence of the fast and slow moving electrons may bealtered; that is to say, the convergence of the fast moving electronsmay be increased by a larger amount than the slow moving electrons.Thus, by a proper selection of potentials applied to the mirror and lenselements, electrons of two difierent speeds may be made to converge upona single point on the axis of the system to produce a true image of theobject point. (Electrons from object points which are off the axis ofthe system are similarly focused.)

. In the drawing, the path a: of those electrons from a central imagepoint on electrode I! which have substantially zero axial velocity allconverge upon point Q on screen 3|. The line 312 represents thetrajectories of electrons from the same object point which have an axialvelocity of 1 volt. These electrons, by the action of the mirror and thelens system (including the external magnet) are made to converge uponthe same point, Q. It should be noted, however, that the path 111 ofelectrons having lower initial velocities does not converge exactly uponpoint Q because the mirror can be made to perfectly correct for only twovalues of initial velocity.

We are aware that others have previously proposed the use of anelectron-mirror or mirrors in a Braun tube to increase the length of thepath of the electron beam and to thereby increase the sensitivity of thedevice. However, such use is not to be confused with our use ofelectron-mirrors for the purpose of curing image defects which occur inthe production of extended or entire electron-images.

What is claimed is:

1. An electron-image device comprising a' an image-target electrode eachhaving a surface 7 area corresponding substantially to the area of saidimage-cathode, means for directing said electron-image from said cathodeto said intensifying electrode, means for directing a secondaryelectron-image from said intensifying electrode to said electron-mirror,and means for directing said secondary-electron image to said targetelectrode.

2. An image intensifier device, comprising an image cathode, asecondary-electron emissive image intensifying electrode, an electronmirror and an image target electrode circumferentially arranged about acommon center, and magnetic means for directing an electron image fromsaid cathode to said image intensifying electrode, thence to saidelectron mirror and finally to said image target electrode.

3. An image intensifier, comprising an evacuated envelope having acentral globe-like portion and a plurality of radially extending neckportions, a photosensitive image cathode and a plurality of electronlens elements in one of said neck portions, a secondary-electronemissive surface and a plurality of electron lens elements in another ofsaid neck portions, a plurality of electron-mirror elements in a thirdneck portion and an image-target in a fourth neck portion, and magneticmeans exterior of said envelope for directing the photoelectron imageformed by said first-mentioned electron-lens elements in a curved pathto said secondary-electron emissive surface, and for directing thesecondary-electron image formed by said second-mentioned electron-lenselements successively to said electronmirror and target electrodes.

EDWARD G. RAMBERG. GEORGE A. MORTON.

